This invention relates to X-ray focusing devices for use with X-ray generators and in particular to X-ray focusing devices which utilise capillary and polycapillary lenses in combination with X-ray focusing mirrors for the close coupled focusing of X-ray beams.
The majority of X-ray generators produce X-ray beams which have a relatively large focal spot or line which requires that the generator utilises a relatively small aperture to restrict beam diameter and divergence. However, the use of small apertures results in a large loss of X-ray intensity.
It is known that X-ray focusing mirrors may be used in order to focus and thereby increase the intensity of the beam from an X-ray generator. An example of such a focusing mirror is that distributed by Bede Scientific Instruments Ltd under the Trade Mark xe2x80x9cMicromirrorxe2x80x9d. xe2x80x9cMicromirrorsxe2x80x9d are now in commercial production and are being used in X-ray generators. The brightness achieved by using the xe2x80x9cMicromirrorxe2x80x9d is comparable to that given by rotating anode generators with total reflection optics.
This focusing mirror comprises a cylindrical body having an axially symmetrical passage extending therethrough. There is an aperture at each end of the body which communicates with the passage. The passage has a profile which may be ellipsoidal or paraboloidal in longitudinal section, depending on requirements. An ellipsoidal profile produces a focused beam with varying divergence and focused spot size, while a paraboloidal profile produces an almost parallel, essentially non-divergent beam. The interior reflecting surface is coated in an exceptionally smooth coating of gold or similar in order to provide specular reflectivity. Typically the mirror is made of nickel and is of the order of 30 mm in length. The outside diameter of the mirror is typically 6 mm. The entry aperture is generally smaller than the exit aperture.
It is known to use capillary lenses to focus X-rays. A capillary lens conventionally comprises a number of capillary tubes bundled together. A capillary lens is capable of focusing X-ray radiation to a small diameter spot, but suffers from the disadvantage that the focused beam has relatively high divergence. In contrast an X-ray mirror can produce a beam of relatively low divergence.
In conventional use, a single X-ray focusing mirror is used to focus the source beam and thus produce a gain in intensity from the X-ray generator to the specimen. However X-ray generators provide X-ray beams which have a relatively large focal spot and therefore even when focused by the X-ray focusing mirror the beam will not be as intense as it can be. In addition, tests have shown that the smaller the dimension of the focal spot the greater increase in gain there will be through the X-ray focusing mirror. Thus, the present invention aims to provide apparatus which in combination will provide an input focal point at the entry aperture of the X-ray focusing mirror which has a diameter as close as possible to zero, thereby maximising the gain through the X-ray focusing mirror to the target specimen.
According to a first aspect of the present invention there is provided an X-ray focusing device comprising a capillary waveguide arranged on a first axis closely coupled to an X-ray focusing mirror, whereby the mirror comprises an interior reflecting surface having a rotational axis of symmetry on a second axis, said first and second axes being substantially collinear.
It will be understood to those skilled in the art that close coupling involves arranging the components of the focusing device such that the separation between them is of the order of magnitude of the length of each component or less, preferably less than 50 mm, most preferably less than 10 mm.
Preferably said interior reflecting surface is ellipsoidal, paraboloidal or conical in longitudinal section.
Preferably said capillary waveguide comprises one or more tapered capillaries arranged symmetrically about said first axis. Preferably the angle of taper of said tapered capillaries is less than 10 mrad.
Preferably the capillary waveguide is arranged to produce a focused X-ray beam of less than 10 xcexcm diameter.
According to a preferred embodiment the capillary lens comprises a single tapered capillary having an internal profile adapted to reduce the diameter of the focal spot of an X-ray source.
According to a second aspect of the present invention there is provided an X-ray focusing device comprising a polycapillary lens arranged on a first axis closely coupled to an X-ray focusing mirror, whereby the mirror comprises an interior reflecting surface having a rotational axis of symmetry on a second axis, said first and second axes being substantially collinear.
Preferably said interior reflecting surface is ellipsoidal, paraboloidal or conical in longitudinal section.
Preferably said polycapillary lens comprises a plurality of tapered capillaries arranged such that both the diameter of the focal spot of an X-ray source and the angular divergence of the X-rays are reduced.
Preferably said capillaries comprises fibres having internal diameters of less than 10 xcexcm, most preferably less than 2 xcexcm.
Preferably said polycapillary lens comprises between 10 and 500, most preferably between 50 and 200 tapered capillaries.
Preferably said polycapillary lens is arranged such that its overall diameter first increases and then decreases with increasing distance from the X-ray source.
Preferably, said mirror is moveable in position relative to said waveguide. Preferably, said device further comprises a guide means for guiding said mirror in a direction parallel to the second axis, and adjustment means for adjusting the spacing of the waveguide and the mirror. Preferably, the device also comprises angular adjustment means adapted to allow angular adjustment of the mirror. Alternatively, said mirror is fixed in position relative to said waveguide.
According to a third aspect of the present invention there is provided an X-ray focusing device comprising a polycapillary lens arranged on a first axis closely coupled to a planar or non-planar X-ray target of an X-ray generator, said polycapillary lens comprising a plurality of tapered capillaries arranged such that the input end of each capillary is arranged substantially normal to the adjacent portion of said X-ray target. The polycapillary lens may be closely coupled to an X-ray focusing mirror at its end remote from the target, in accordance with the first or second aspects of the invention.
Preferably said polycapillary lens is arranged such that its overall diameter first increases and then decreases with increasing distance from the X-ray source.
According to a fourth aspect of the present invention there is provided an X-ray generating device comprising an annular electron source arranged about a tapered or conical X-ray target closely coupled to a polycapillary lens or an X-ray focusing mirror. The X-ray target may be coupled to a polycapillary lens, which is itself closely coupled to an X-ray focusing mirror at its end remote from the target, in accordance with the first or second aspects of the invention.
According to a fifth aspect of the present invention there is provided an X-ray focusing device comprising a substantially hemispherical X-ray target closely coupled to a polycapillary lens or an X-ray focusing mirror, the target comprising a plurality of channels axially orientated towards the hemispherical centre. Preferably the device is positioned such that the electron source is at the hemispherical centre. The X-ray target may be coupled to a polycapillary lens, which is itself closely coupled to an X-ray focusing mirror at its end remote from the target, in accordance with the first or second aspects of the invention. Preferably the lens or mirror is arranged such that the angle of collection of the lens or mirror is the same as the angle subtended by the hemispherical target at the hemispherical centre.